The present invention relates to a confocal optical system installed in a scanning confocal endoscope which enables observation of tomograms of a tissue in a body cavity.
Confocal probes employing an optical system of a confocal microscope (capable of obtaining images of higher magnification and resolution than ordinary endoscope optical systems) as their probes and scanning confocal endoscopes employing such a confocal probe are well known. The confocal probe applies a laser beam as illuminating light to a living tissue (subject) in a body cavity and selectively receives reflected light or fluorescence from the tissue that is on the object-side focal plane of its objective optical system. In other words, the confocal probe extracts light that is on the focal plane from the reflected light or fluorescence from the tissue. Probes are generally classified into direct view probes (applying the illuminating light to the tissue from their tips) and lateral view probes (applying the illuminating light to the tissue from their lateral faces). The type of optical path required varies depending on whether the probe is a direct view probe or a lateral view probe, therefore, the optical system for each probe has to be designed properly depending on the type of the probe.
Optical systems usable for a confocal probe have been disclosed in Japanese Patent Provisional Publication No. 2000-292703 (hereinafter referred to as “patent document #1”) and Japanese Patent Provisional Publication No. 2000-258699 (hereinafter referred to as “patent document #2”), for example. The patent document #1 discloses an optical system usable for a lateral view confocal probe.
The optical system of the patent document #1 scans or dynamically deflects a beam on a living tissue (subject surface) by use of reflecting surfaces of mirrors, for example. Therefore, the distance between the light source and the objective lens (condensing optical system) is set long enough for securing a space for placing the reflecting surface and thereby the diameter of the objective lens is necessitated to be large and attaining a sufficiently wide scan range becomes difficult. Moreover, the space where the reflecting surface is placed is required to further include a space for driving the reflecting surface in a prescribed direction.
Further, in the patent document #1, the aperture stop and the first surface of the condensing optical system are provided with reflecting surfaces and the scanning of the beam is realized while securing a necessary optical path length by letting the beam travel to and fro between the aperture stop and the first surface. Therefore, loss of light quantity occurs on each reflection of the beam and it is impossible to use the beam efficiently. Moreover, the optical system of the patent document #1 does not have a mechanism for changing the condensing position (focal position) of the beam in the depth direction of the subject, that is, in the optical axis direction of the objective lens. Thus, it is impossible to detect the position of the subject surface accurately and obtain high definition images of the subject.
By the way, in recent years, a scanning confocal endoscope of an integrated type having functions of a conventional endoscope and a confocal probe (hereinafter simply referred to as an “integrated endoscope”) is being hoped for in order to reduce the load on operators for handling probes. Such an integrated endoscope is required to include both a first optical system for general endoscope observation (hereinafter called “general observation”) and a second optical system for the so-called “confocal observation”, independently. Therefore, reducing the diameter of the flexible tube (endoscope) by miniaturizing each optical system (especially, the optical system for the confocal observation) becomes the most critical challenge. However, application of the composition of the above patent document #1 or #2 to the confocal observation optical system causes increased diameter and length of the flexible tube. Meanwhile, for realizing widest-range scanning of the beam while miniaturizing the flexible tube, it is possible to let the confocal observation optical system adopt an optical system which scans a beam spot on the subject surface by swinging the point source of light (U.S. Pat. No. 5,161,053, for example).
However, there have been no concrete discussions or propositions on a condensing optical system that is suitable for the case where the swinging of the point source of light is employed for the optical system for the confocal observation, that is, a condensing optical system for a confocal observation optical system capable of satisfactorily reducing the loss of light quantity and suppressing various aberrations.